Just how accurate are the speed monitoring devices that are used?

This is a question to patrol officers. Just how accurate are the speed monitoring devices that are used? I have received a ticket or two in the past for breaking the speed limit. I have never thought to question the accuracy of the devices that are used to record said speed. When writing a citation for speeding do you have to back up the recorded speed with some type of computer records, or is it just officer word? This is just something that I have always wondered about and would appreciate any input. Thanks so much.
 
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